1. Field of the Invention
The present invention relates to a mobile radio terminal a communication processing method, and particularly to a mobile radio terminal and a communication processing method capable of connecting to a circuit switching (CS) access network if the mobile radio terminal receives a CS service event while being on standby in an Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
2. Description of the Related Art
The 3GPP is a standardization body for standardizing third-generation cellular phone standards. The 3GPP TS23.272 V8.0.0 (hereinafter referred to as “TS23.272”) developed by the 3GPP defines a method of cooperative operation (CS Fallback) performed if, in an Evolved Packet System (EPS) integrating an E-UTRAN and a mobile communication network having a CS network, a CS service is carried out while a mobile radio terminal (user equipment (UE)) is on standby in the E-UTRAN. Examples of the mobile communication network include a Universal Terrestrial Radio Access Network (UTRAN), a GSM/EDGE Radio Access Network (GERAN), and a cdma2000 1x CS access network (hereinafter referred to as “1x CS access network”).
FIG. 1 illustrates an overview of an EPS having a CS Fallback function. As illustrated in FIG. 1, the EPS has an E-UTRAN, a UTRAN, and a GERAN. The UTRAN and GERAN serve as access networks that provide CS services. The E-UTRAN, UTRAN, and GERAN are connected, through their respective interfaces, to a Mobile-services Switching Center (MSC) Server within a Core Network (CN). The EPS is configured to have the E-UTRAN, UTRAN, and GERAN, the latter two serving as access networks that provide CS services, but may be configured to have a 1x CS access network.
In an EPS having a CS Fallback function, a mobile radio terminal performs a standby operation typically in an E-UTRAN. If a CS service event, such as a CS incoming call notification from the EPS or a service request within the mobile radio terminal (e.g., CS outgoing call, CS incoming call, or CS Short Message Service (SMS)), occurs, the mobile radio terminal connects to a UTRAN and a GERAN, or to a 1x CS access network to perform the CS service. Then, the mobile radio terminal transmits and receives information to and from the network. Sections 6.4 and 7.2 describe processing to be carried out if a mobile radio terminal that is on standby in an E-UTRAN base station performs outgoing and incoming call operations on a UTRAN or GERAN that provides CS services. FIG. 2 illustrates a sequence in which a mobile radio terminal that is on standby in an E-UTRAN base station receives a CS incoming call and connects to a UTRAN or a GERAN. Also, a connection to a 1x CS access network is described in Annexes B2.2 and B2.3.
FIG. 3 illustrates a processing sequence performed between a mobile radio terminal and each component of an EPS if the mobile radio terminal which is on standby in an E-UTRAN transmits an outgoing call signal to a 1x CS access network. FIG. 4 illustrates a processing sequence performed between a mobile radio terminal and each component of an EPS if the mobile radio terminal which is on standby in an E-UTRAN processes an incoming call from a 1x CS access network.
According to TS23.272, if being on standby in an E-UTRAN and using a CS Fallback function, a mobile radio terminal can use CS services not directly provided by the E-UTRAN, without having to be on standby in a CS access network.
According to the 3GPP TS23.272 V8.0.0, a mobile radio terminal which is on standby in an E-UTRAN does not start connecting to a 1x CS access network until a CS service event occurs. Generally, as long as being on standby only in the E-UTRAN, the mobile radio terminal does not have to be on standby in the 1x CS access network. Therefore, only when necessary, the mobile radio terminal activates a circuit for an access network other than the E-UTRAN within the mobile radio terminal, and performs synchronization, reception of notification information, and message transmission and reception. Thus, the state where the mobile radio terminal is on standby only in the E-UTRAN is preferable in that it is possible to reduce power consumption.
However, since network synchronization is not achieved between an E-UTRAN and a CS access network, such as a UTRAN, it is not possible to determine the synchronization timing of the CS access network on the basis of that of the E-UTRAN. Therefore, if the mobile radio terminal receives a request for a CS service, it is necessary to acquire synchronization with the UTRAN. As a result, it takes long time to start the CS service. The same problem occurs in the case of the E-UTRAN and the 1x CS access network.
Hereinafter, the above problem will be described in detail. FIG. 2 illustrates a sequence performed if a mobile radio terminal which is on standby in an E-UTRAN recognizes a CS incoming call. Here, the mobile radio terminal is not on standby in a CS access network (UTRAN, GERAN, or 1x CS access network). The mobile radio terminal starts connecting to the CS access network only after receiving a notification of a CS incoming call (Paging) (see “6” of FIG. 2). FIG. 5 illustrates sequential processing performed in a mobile radio terminal if a notification of a CS incoming call (Paging) is received. Here, the mobile radio terminal includes an E-UTRAN L1 unit having a function of connecting to an E-UTRAN and a 1x CS access network L1 unit having a function of connecting to a 1x CS access network. The E-UTRAN L1 unit and the 1x CS access network L1 unit are based on lower-level protocols in a radio interface, and their processing is controlled by a communication-control central processing unit (CPU) of the mobile radio terminal.
As illustrated in FIG. 5, the mobile radio terminal uses the E-UTRAN L1 unit to be on standby only in the E-UTRAN. The mobile radio terminal uses the E-UTRAN L1 unit to perform a intermittent reception operation on a Page Indicator Channel. As illustrated in FIG. 5, an E-UTRAN base station transmits a Page Indicator, using a CS Fallback function, to communicate the presence of a CS incoming call. Upon recognizing that there has been a notification of a CS incoming call, the E-UTRAN L1 unit of the mobile radio terminal notifies a CPU that there has been a notification of a CS incoming call. To enable the 1x CS access network L1 unit to perform radio processing etc. to receive the CS incoming call, the CPU switches from the E-UTRAN L1 unit to the 1x CS access network L1 unit, and activates a radio unit etc. to allow communication in the 1x CS access network.
However, in order to return a response to the CS incoming call (i.e., to return a Paging response) to the 1x CS access network, the 1x CS access network L1 unit of the mobile radio terminal needs to search for a base station to acquire synchronization and to receive notification information. The time required to perform this series of processes may vary depending on the configuration of the mobile radio terminal, but it may take several seconds including processing overhead between radio software programs in the mobile radio terminal. Moreover, if it takes long time from receipt of Paging to returning a Paging response, a timeout of a timer waiting for a Paging response from the mobile radio terminal occurs in the EPS. As a result, the mobile radio terminal may not be able to receive an incoming call. In the case of another CS service, such as a CS outgoing call, if an outgoing call operation for making an emergency outgoing call is delayed for the same reason as that described above, the emergency outgoing call cannot be made in the worst case, which may cause discomfort to the user.